This invention relates to improvements in a magnetic recording medium, and more specifically to improvements in a back coating to be formed on the back of the base for the magnetic recording medium.
The magnetic recording medium is in extensive use in audio and video recording and also in computer and other fields. Especially, video tape cassettes (e.g., of the VHS and beta systems) have enjoyed widespread use in recent years. Consequently, intensive and earnest studies are being carried on to realize commercial production of video cassette tapes capable of recording and playing for long periods of time, in the same manner as thin audio cassette tapes have successfully been merchandised for extended recording and playing time.
For the tape performance the characteristics of the magnetic recording layer formed on one side of a tape base is, of course, a chief determinant. Also, it is to be noted that the characteristics of the back surface of the base are important considerations in use of the tape and from the standpoint of overall tape characteristics. Because of the trend towards adoption of ever thinner tape bases, forming a thin coating on the back of the tape for the reinforcement purpose has been proposed. However, among others the following matters become a problem:
1. Video S/N PA0 2. Cinching (loosening of tape roll on quick stop) PA0 3. Wear of back coating PA0 4. Mutual adhesion of magnetic layer and back coatings on adjacent layers of wound tape
The back coating is required to have these characteristics in a balanced way. Heretofore, adequate consideration has seldom been given to the characteristics in general, and no magnetic recording tape has been introduced yet which has a back coating with well-balanced characteristics.
In view of the above, we are proposing a magnetic recording medium having a back coating which obviates the drawbacks of the prior art magnetic recording medium and exhibits very advantageous effects. The magnetic recording medium with desirable characteristics is obtained, for example, by dispersing a nonmagnetic powder in a binder which is a mixture of a thermosetting resin, typically represented by a vinyl chloride-vinyl acetate copolymer, polyurethane, and an isocyanate compound, with or without the further addition of nitrocellulose, and then applying the whole mixture on a base to form a back coating thereon. Some other proposals of the character have also been made.
Basically, these techniques may be boiled down to a common method of forming magnetic recording layer on one side of a base and forming a back coating on the other side, the back coating material consisting of a nonmagnetic powder dispersed and mixed in a binder composed chiefly of a thermosetting resin. As compared with the ordinary magnetic recording medium free of back coating, such a back-coated one has presented a new problem in addition to those referred to above. It is jitter, a phenomenon of video signal instability caused by slight phase variations. The phenomenon is believed to be related to the smoothness of tape running.
Our investigations about the way of preventing the jitter have now revealed that the addition of a lubricant which consists of a fatty acid or a fatty ester to the back coating material is effective for achieving the end. The fatty acid or ester thus incorporated proves highly jitter-preventive without sacrificing other characteristics required of the back coating.